1. Field of the Invention
The present invention is directed to the field of internal combustion engine control systems and primarily to that portion of the above-noted field which is concerned with the chemical analysis of the composition of the exhaust gases produced by the internal combustion engine. More particularly, the present invention is concerned with an electrical system for maintaining the temperature of an exhaust gas sensor at a closely controlled value so that signals produced by the sensor may be accurately and reliably related to the concentration of oxygen in the exhaust gases. The present invention is also concerned with a system for providing an exhaust gas sensor output signal which is linear with respect to changes in the air/fuel ratio of the combustible mixture being provided to the engine and producing the exhaust gas environment of the sensor.
2. Description of the Prior Art
Ceramic sensors for electrically reacting to the partial pressure of oxygen within the exhaust gases produced by an internal combustion engine are known. One such sensor is described in co-pending, commonly assigned patent application Ser. No. 391,424 -- "Air Fuel Ratio Sensing System" and filed in the names of H. L. Stadler et al. The sensor there described relies upon changes in the electrical resistance of titania ceramic material in response to changes in the partial pressure of oxygen in the environment of the sensor. These sensors operate at elevated temperatures on the order of about, for example, 600.degree. to 900.degree.C. Other sensor ceramic materials having a variable electrical resistance are known. Since the resistance of the ceramic material may vary with variations in the temperature within the operating range, as well as with variations in the partial pressure of oxygen, it has become apparent that accurate control of the temperature of the sensor is desirable when partial pressure of oxygen determination is desired.
One prior art system relies upon a platinum heater wire embedded within the ceramic element and an embedded thermocouple operating a control system to maintain a substantially constant flow of electrical current of variable magnitude through the heater wire consonent with maintaining the temperature of the ceramic material at a selected level. By way of example, temperatures of 700.degree.C., plus or minus 2.degree.C are desired so that the electrical resistance of the sensor ceramic may be directly translated into a partial pressure of oxygen for the exhaust gases and, concomitantly, an accurate measure of the air/fuel ratio of the combustion mixture generating the exhaust gases.
This structure, that is, one in which an embedded thermocouple is used to variably control the current through a heater winding, has produced a variety of problems. Firstly, the embedded thermocouple, if it is to be accurate within the desired range of accuracy, is an expensive element. Secondly, the continuous control of a variable flow of electrical current through a heater winding requires expensive electronic or electromechanical components and results in consumption of electrical energy which occurs remote from the sensor and which is wasteful. It is therefore a specific object of the present invention to provide a system for controlling the heating of a wafer of ceramic material which does not require the use of a thermocouple. It is also an object of the present invention to provide such a system which does not dissipate substantial amounts of electrical energy in structures remote from the sensor ceramic. More particularly, it is an object of the present invention to provide such a system which dissipates only minor amounts of electrical energy in components other than the heater.
In the control of an internal combustion engine to provide an exhaust gas having a precisely controlled chemical composition for subsequent treatment by exhaust gas treatment devices, the maintenance of the air/fuel ratio of the combustion mixture at a precisely controlled value is of cardinal importance. The known exhaust gas sensor ceramic materials which demonstrate a variable resistance in response to variations in the partial pressure of oxygen in the exhaust gases and which are compatible in terms of response time and life capacity with an automotive environment show a resistance variation such that the logarithm of resistance is approximately linear in terms of variation in air/fuel ratio. In order to provide an output signal for modulating either the air or the fuel content of the air/fuel mixture it is desirable to match the resistance variation of the sensor, the output signal, and the response of the modulating mechanism. It is therefore an object of the present invention to provide an electrical system for generating an exhaust gas sensor output signal which varies approximately linearly with respect to variation of the air/fuel ratio of the combustion mixture. More particularly, it is an object of the present invention to provide an electrical system for converting the normally logarithmic sensor signal to an approximately linear sensor signal for use by a controller having an approximately linear response for small sensor signals for modulating either the air or the fuel content of the combustion mixture.